[Hormone therapy as postoperative care in patients with differentiated thyroid cancer]

TSH suppression therapy has been performed for 150 patients as postoperative care for differentiated thyroid cancer. L-thyroxine (LT4) was given in a dose of 2.1-4.8 micrograms/kg/day, per os. Serum thyroid hormone determinations were done after confirming that the drug had continually been taken for more than three months. Serum free T4 (FT4) values were elevated higher than the upper normal limits (1.9 ng/dl) in 127 of 150 cases (84.7%). In 100 cases of these, serum free T3 (FT3), reverse T3 (rT3) and TSH were estimated. In 98 cases (98%), serum FT3 was within the normal limits (3-6 pg/ml) and no correlation was found between values of FT4 and FT3. These serum rT3 values, however, were elevated higher than the normal range (13-43 ng/dl) in 41 cases (41%) correlating with FT4 values. Serum TSH, FT4 and FT3 showed no correlation with given LT4 doses. There was no correlation between the serum TSH and FT3. However, some correlation (r = -0.4046) was found between serum TSH and rT3 values. From these results the following conclusions were obtained: i) There must be some auto-regulating mechanisms to keep the serum FT3 values at the normal level by promoting a de-iodinating process from T4 to rT3 when serum T4 level increased, since rT3 has no hormone activity. ii) Administration of synthesized LT4 is safer and more adequately usable than synthesized L-triiodothyronine or desiccated thyroid, as the TSH suppression therapy, because the above mentioned autoregulating mechanism would take place.(ABSTRACT TRUNCATED AT 250 WORDS)